1. Technical Field
The present invention relates to a fixed type constant velocity universal joint which is used, for example, for a drive shaft of a power transmission device for automobiles and industrial machines.
2. Background Art
As illustrated in FIG. 5, a fixed type constant velocity universal joint includes an outer joint member 3 having an inner surface 1 provided with a plurality of track grooves 2, an inner joint member 6 having an outer surface 4 provided with a plurality of track grooves 5 paired with the track grooves 2 of the outer joint member 3, a plurality of balls 7 for transmitting torque while being interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6, and a cage 8 for holding the balls 7 while being interposed between the inner surface 1 of the outer joint member 3 and the outer surface 4 of the inner joint member 6. The cage 8 includes a plurality of window portions 9 for housing the balls 7, which are arranged along a circumferential direction.
Further, the outer joint member 3 includes a mouth portion 3a provided with the track grooves 2, and a stem portion 3b projected from a bottom wall 10 of the mouth portion 3a. Female splines 11 are formed in an inner surface of an axial hole of the inner joint member 6, and an end portion of a shaft 12 is fitted to the axial hole of the inner joint member 6. The end portion of the shaft 12 includes male splines 13, and the male splines 13 are fitted to the female splines 11 of the axial hole of the inner joint member 6. Note that, a circumferential groove is formed across end portions of the male splines 13, and a stopper ring 14 as a retaining element is fixed in the circumferential groove.
In this case, the track grooves 2 of the outer joint member 3 each have a center of curvature O1 offset from a joint center O to an opening side of the outer joint member 3 by a predetermined dimension along a joint axial line L in an axial direction. Further, the track grooves 5 of the inner joint member 6 each have a center of curvature O2 offset by a predetermined dimension along the joint axial line L in the axial direction from the joint center O to a depth side opposite with respect to the center of curvature O1 of each of the track grooves 2 of the outer joint member 3. In other words, the center of curvature O1 and the center of curvature O2 are offset from the joint center O to sides opposite to each other by an equal distance along the joint axial line L in the axial direction.
An opening portion of the constant velocity universal joint is sealed with a boot 15. The boot 15 includes a large diameter portion 15a, a small diameter portion 15b, and a bellows portion 15c for coupling the large diameter portion 15a and the small diameter portion 15b to each other. The large diameter portion 15a is externally fitted to a boot fixing portion 17 on the opening portion side of an outer surface of the outer joint member 3, and the large diameter portion 15a thus externally fitted is fastened with a boot band 16. In this way, the large diameter portion 15a is fixed to the boot fixing portion 17 of the outer joint member 3. Further, the shaft 12 is provided with a boot fixing portion 18 provided with a circumferential groove. The small diameter portion 15b of the boot 15 is externally fitted to the boot fixing portion 18, and the small diameter portion 15b thus externally fitted is fastened with another boot band 16.
By the way, in the field of transportation in which CO2 reduction is strongly desired, in particular, with regard to automobile components, longer duration of use and weight reduction have been urgent problems in terms of life cycle assessment (LCA). In order to prolong duration of use, it is particularly necessary to prolong lives of rolling elements, and in order to achieve weight reduction, it is necessary to increase permissible loads of the components under loads. Therefore, there has been a strong demand to prolong the lives of rolling elements.
Further, a half shaft, which is one of drive shafts of an automobile, is used under the following conditions: a condition of low speed rotation at high torque at the time of, for example, slope climbing (hereinafter referred to as high load condition); a condition of high speed rotation at low torque at the time of, for example, traveling on a flat expressway (hereinafter referred to as low load condition); and a condition of creep rotation at low torque at the highest steering angles of wheels (near maximum operating angles of CVJs) (hereinafter referred to as high angle condition). Such typical load modes are repetitively switched to one another, and hence a fixed type constant velocity universal joint using balls as torque transmitting members (hereinafter sometimes referred to as ball fixed type constant velocity universal joint) needs to be sufficiently durable in all the traveling modes. Thus, even when a ball fixed type constant velocity universal joint is excellent in durability under only one of the above-mentioned conditions, such a ball fixed type constant velocity universal joint is insufficient in practical durability. Further, a plunging type constant velocity universal joint is used at operating angles lower than (approximately half of) those of the ball fixed type constant velocity universal joint, and hence, in many cases, can be used for a longer period than that for the ball fixed type constant velocity universal joint. In view of the circumstances, it is particularly desired to prolong lives of rolling elements of the ball fixed type constant velocity universal joint.
Durability cannot be enhanced without consideration of lubricant. A composition of the lubricant is significantly different depending on a product to be used. This is because tribological phenomena significantly vary from each other depending on motional states of components and properties of surfaces, the components and the surfaces being brought into contact with each other. Thus, in order to determine the composition of the lubricant, it is necessary to definitely limit a product in which the lubricant is sealed (for example, ball fixed type constant velocity universal joint for half shafts of automobiles), grasp driving conditions, and take the surface properties of rolling surfaces into consideration.
JP 3988895 B, JP 2004-123858 A, and JP 3988897 B each describe an example of the lubricant to be sealed in constant velocity universal joints. According to the grease compositions described in JP 3988895 B and JP 2004-123858 A, abrasion of constant velocity universal joints is effectively reduced, and occurrence of flaking at lubricating parts is prevented. However, the grease compositions provided in those cases are effective only under the high load condition. Meanwhile, JP 3988897 B describes a grease composition which reduces abrasion of constant velocity universal joints and prevents generation of vibration.
Further, JP 61-12791 A describes a grease composition which improves at least one of the following lubrication parameters: load resistance; abrasion resistance; frictional coefficient; and the like. The grease composition, which is improved in at least one of those parameters, can be effectively used for lubricants for various machines and apparatus poor in those parameters.
However, none of JP 3988895 B, JP 2004-123858 A, and JP 3988897 B above describes surface properties of a contact surface. Practically, tribological phenomena significantly vary from each other depending on motional states of components and properties of surfaces, the components and the surfaces being brought into contact with each other. Thus, in a fixed type constant velocity universal joint which uses balls and is used for a half shaft of an automobile, it is necessary to determine the composition of the lubricant based on driving conditions and in consideration of surface properties of driving surfaces. Further, the grease composition according to JP 61-12791 A is not limited to use for constant velocity universal joints, or not limited at all in terms of the surface properties of the contact surface.